fixed audio source crashing if no device is available

Author: AlexandreRouma

source_modules/audio_source/src/main.cpp

@@ -183,6 +183,9 @@ class AudioSourceModule : public ModuleManager::Instance {
     static void start(void* ctx) {
         AudioSourceModule* _this = (AudioSourceModule*)ctx;
         if (_this->running) { return; }
+
+        // If no device is selected, give up
+        if (_this->selectedDevice.empty()) { return; }
 
         // Stream options
         RtAudio::StreamParameters parameters;